Antioxidants and Performance: What Does Science Say?

Author: Pedro Valenzuela from Fissac.com, Researcher in the Physiology Unit of the University of Alcalá and in the Performance Control Unit at the Sports Medicine Center (AEPSAD, CAR of Madrid)

When we hear 'oxidative stress,' it is normal to tend to associate it with something negative. How many times have we heard that actions like smoking increase oxidative stress? Or that aging accelerates due to an excess of oxidative stress? And the truth is that in large part it is like that: an excess of free radicals and, consequently, elevated levels of oxidative stress can cause cellular damage.

That is why, for decades, the consumption of antioxidants (substances that help fight oxidative stress, such as vitamins A, C, or E, or many other elements like beta-carotene or flavonoids) has been promoted for virtually any type of population, to the point that currently supplements with antioxidants (or even foods enriched with them) are still among the most marketed.

In the field of exercise, antioxidants are also very popular, and they are present in a large number of nutritional supplements. However, before discussing their effectiveness, we must understand what role oxidative stress plays when we exercise.

Exercise and oxidative stress: the benefits of antioxidants

Although performing exercise regularly leads in the long term to a greater endogenous antioxidant capacity, in each training session the production of free radicals increases due to different mechanisms such as the higher activity of the respiratory chain in the mitochondria, small microtears at the muscle level, and other processes in the cytoplasm of muscle fibers.¹ As we have mentioned, it is partly understandable that we tend to associate this increase in free radicals with negative processes. In fact, high levels of oxidative stress, such as those that can occur after very intense, prolonged, and/or exhausting exercises, are partly responsible for muscle fatigue and post-exercise muscle damage (commonly known as delayed onset muscle soreness).²

In this regard, there is evidence showing that the consumption of antioxidants can to some extent prevent these consequences. For example, already 30 years ago it was demonstrated in humans that the consumption of antioxidants (N-acetylcysteine) improved muscle performance during a session of muscle electrostimulation.³ Years later, a study led by the Spanish researcher Mari Carmen Gómez-Cabrera and published in the prestigious journal JAMA evaluated the effect of supplementation with another antioxidant (allopurinol) in professional cyclists during the Tour de France.The authors observed that those who took allopurinol before each stage had lower values of muscle damage indicators (creatine kinase) compared to the cyclists who had taken a placebo.⁴ A decade later, evidence continued to accumulate regarding the possible benefits of antioxidants on performance. For example, a study conducted on triathletes showed that the consumption of antioxidants over 9 days improved performance in a simulated cycling competition, as well as reduced levels of inflammation and oxidative damage.⁵

Oxidative stress and training adaptations: the dark side of antioxidants

However, although excessive oxidative stress can have negative consequences acutely for performance, it is important to understand that oxidative stress is something physiological and necessary for the proper functioning of the body. In fact, oxidative stress is a key player in training adaptations. Like a vaccine, the accumulation of episodes of oxidative stress in each training session induces a greater long-term antioxidant capacity, and acts as a switch for some beneficial adaptations to occur at the muscle level.

That was the finding of Dr. Gómez-Cabrera already 20 years ago. In a study conducted on rats and published in the Journal of Physiology, the authors observed that acute exercise induced oxidative stress. However, this in turn activated a series of signals that promoted both the antioxidant response and muscular and vascular adaptation, and the consumption of antioxidants (allopurinol) inhibited these signals.⁶ In line with these results, years later the same research group observed both in rats and humans that the intake of antioxidants (in this case, vitamin C) decreased the improvements in performance that were obtained after a training period.⁷ In addition, in the animals biochemical analyses were performed and it was observed that antioxidants prevented the increase ofmarkers of mitochondrial biogenesis (e.g. PGC1a), that is, of improvement in the quantity and quality of mitochondria.

Years later, the evidence regarding the possible harmful effects of antioxidants on training adaptations has continued to accumulate. For example, various studies in humans have shown that the habitual intake of antioxidants (in this case, vitamins C and E) attenuates the activation of mitochondrial biogenesis in response to resistance training,⁸ or even decreases strength gains after 10 weeks of this type of exercise training.⁹ Therefore, it seems clear that caution should be exercised when regularly supplementing with antioxidants.

Conclusions

As we can see, antioxidants can be a double-edged sword when it comes to sports performance. On one hand, the excess oxidative stress that occurs during an exhausting exercise session can be a mechanism of fatigue. Based on this, the consumption of antioxidants may be advisable during sessions in which one wants to maximize sports performance without considering the adaptations resulting from that session, such as in a one-stage or multi-stage competition. However, the regular intake of antioxidants can attenuate the adaptations triggered during the training process (which are partly induced by oxidative stress), thus limiting the benefits obtained (e.g., muscle adaptations and mitochondrial biogenesis).